*12* is chamber music work composed with the goal of letting audience members have an engaging, individualized, and influential role in live music performance using their mobile phones as custom tailored musical instruments. The goals of direct music making, meaningful communication, intuitive interfaces, and technical transparency led to a design that purposefully limits the number of participating audience members, balances the tradeoffs between interface simplicity and control, and prioritizes the role of a graphics and animation display system that is both functional and aesthetically integrated. Survey results from the audience and stage musicians show a successful and engaging experience, and also illuminate the path towards future improvements.